Plastics are used in many fields such as the automotive industry, mainly due to their great strength and low weight. Special demands are required for screw connections used to obtain a detachable connection of plastic components. Conventional sheet metal or wood screws do not currently meet these demands. Thread-forming or thread-rolling screws are generally subsumed under the term “wood screws.” These screws are comprised primarily of a head and a screw core or shaft in the form of an elongated cone or pin that tapers to a point at the tip of the screw. The screw core is surrounded by a helical peripheral cutting edge projecting radially from the thread root, which transitions into the two flanks of the cutting thread with a sharp bend. The flanks in turn are provided with a constant pitch from the thread root to the crest, resulting in a uniform thread angle between the two flank lines. Screws of this kind are also used to screw plastics together.
In addition, competition focusing on light-weight construction in the automotive industry has resulted in an ever-growing range of applications for plastics, including increasing use of physical or chemical foams which replace the plastic material with air (for e.g., nitrogen or carbon dioxide). In the case of physical foams produced using MuCell® or similar processes, substances such as nitrogen, carbon dioxide or a chemical blowing agent are injected under pressure in a supercritical state into the molten plastic and uniformly distributed. After injection into the unpressurized mold, the gas separates from the smelt and forms a fine-celled foam structure. The omission of holding pressure and the reduced viscosity, minimizing or eliminating sink marks and strain are the primary aspects in support of this trend.
When used in plastics, including foamed plastics, prior art screws suffer from various drawbacks. For example, they damage the plastic they are driven into, for instance, by rupturing the screw-in openings or by stress cracking. At the same time they also require relatively high screw-in torque. In screw connections using MuCell® injection molded parts, the compact layer (i.e. the outer layer of the component) is destroyed, resulting in the conventional screw completely losing its hold in the remnants of the foam layer.
Additional methods for attaching plastic parts are discloses in German Patent Document DE 100 48 975 C1, German Patent Document DE 10 2009 024 264 A1, and European Patent Document EP 2 185 828 B1. For example, DE 100 48 975 C1 discloses a nut of foamed POM with a cylindrical inner bore, that is screwed onto a threaded bolt. The nut in this case may elastically compress its way through the foamed material, or the thread of the bolt may press into the nut when the nut is screwed on. DE 10 2009 024 264 A1 discloses a nut with sections that have different inner diameters in order to prepare it for use with threaded bolts that have different outer diameters. EP 2 185 828 B1 describes a screw with truncated thread crests and acute thread base angles to produce a thread the first time the screw is driven in.
German Patent Document DE 10 2014 114 165 discloses a screw for integral foamed parts that is suitable for non-thread-rolling use in a tube.